Squirrel-cage pump motor or the like



Aug. 9, 1932.

F. J. JOHNS SQUIRREL CAGE PUMP MOTOR OR THE LIKE Filed Oct. 22, 1929 fies/'sfance Ref/0s.

Raf/'05 of Bar Resisfances a) J/arfiny and Hill Speed Raf/0s of Indacfances.

Jooo moon /s:ooo 'zaaoo 2.50

m. conducfors per 6/0/1 Absc/lssas are X dZ g (lb/babes).

INVENTOT? Francis J Jafins.

ATTORNEY Patented Aug. 9, 1932 UNITED STATES. JPA

TENT QFFIECE FRANCIS J. JOHNS, orwrmzmsnuae, PENNSYLVANIAQASSIGNLOR TO wn's rinenousn LLnorR-Ic & MANUFACTURING COMPANY, A conronarron or PENNSYLVANIA sQUIRR L-CAGE PUMP MOTOR onrnn LIKE v Application filed October 22, 1929. Serial No.'401,574;

My invention relates to a rotor slot arrangement for reducing the starting currents in a motor in which it is desired to have not only a reasonably good startingtorque but also a good performance under running conditions; and it has more particular reference to such motors for driving centrifugal pumps, which do not require exceptionally high starting torques because a centrifugal pump does not take its load until it has come up to speed.

My invention has particular relation to deep-bar squirrel-cage motors in which the eddy currents in the squirrel-cage bars affect the starting conditions by crowding the secondary currents to thetops of thebars at starting, thereby increasing the resistance of the squirrel-cage at starting. The effect of the eddy currents in the deep barsis-not only to increase the starting resistance, which is desired, but to decrease the inductance, because the bar acts, on starting,.as if it were shallower than it really is. The decrease in inductance at starting is detrimental tothe securing of a low value of starting current, since the reactance of the motor is the greater part of its impedance on starting. The increase in bar resistance is advantageous since it increases the starting torque. i'

The standard construction of deep-barmo tors accomplishes the right resistance condi-' tions at starting, but the inductance change is just opposite to the ideal conditions, which would be a motor having a higher inductance at starting than under running conditions.

According to my present-invention, I have approached the ideal condition of a deep-bar;

squirrel-cage motor by keeping the induct, ance at start approximately the same'asthe inductance under load,'the inductance being only slightly higher under load conditions than at start. i

y invention willbe explained after ads scription is given of a preferred embodiment thereof, with reference to the accompanying drawing, wherein g 57 Figure 1 is a fragmentary transverse sectional full-scaledrawing of the rotor member of athreephase, -horsepower, le-pole,

(BO-cycle, 1750 R P. motor embodyingniy invention, Y i

' Fig, 2 is a curve chart hereinafter ex plained, and V i r v v 7 F ig, 3 is ancnlargedview showing one of the rotor slots, with. the conductor therein shown in section. I Y

"As shown in Fig. 1, the steelrotor punch-1 ings 3 of my motor are provided with a plu-- rality of deep perforations or so-called slots 4-in which are disposed'the squirrel-cage barsv 5 which are preferably made of hard-drawn copper and rectangular in cross-section. The bars 5 are buried -inthe rotor'to provide a bridge (lover each of theslots or perforations l, and: each bridge is sjlitted, in aradial direction,by a slit 7 which isas narrow as itis economically practical tomake pit, with a reasonable life of the punching dies, and which extends upwardly from the top of the slotl to within a vfew hundredthsof aninch; of the outer periphery of the rotor punching 3. At present, I am making the slits 31 mils wide, but I contemplate making the slits of even narrower widths. 4 y i The ratio of the bar-resistance at starting and at fullspeed, respectively, is dependentrupon the slot or bar-depth (l, regardless of the bar-width b, and-it isfalso proportional to the square root-ofithe;frequency (usually expressed in'cycles per second) and inversely proportional to the square root of: the resistivity (which may be conveniently expressed in ohms per inch cube). The man;

her in which the ratio of the bar-resistance at starting to the bar-resistance at'full speed varies for different valuesofX= d p is shown in the upper portion of Fig. 2, where the number of superposed conductors per slot is indicated by m. In general, myinvention would beapplied only to fa 'rotor havinga single conductor per slot, following the curve m=1, although, in exceptional cases, the other curves might be followed. i

The ratio of thGSlOt-iIlClUCtfiHCQS at starting and a t full speed are also'dependent upon X =d the manner of- Variati-on being as. compared with hi second factor.

indicated by the curves in the lower portion of Fig. 2.

From the curves in Fig. 2, it' will be seen that, for a rotor bar having three timesas much resistance at starting as at runn ng speed, the starting inductance of the portion of the slot in which the bar is located will he only one-half as much as the running 11 (lnctance. h re t A po n ed out above, this i more of a decr as i the ductanc t starting than is desirable, because of its effect in increasing the starting current-without a proportional increase in'the starting torque. Heretofore, it has been customary, where a larger starting reactance is required in a motor of the character under: consideration, to'add a few more turns of primary winding, which has the effect of decreasing the; flux density and'increasing both the primary and the secondary leakage reactances of the motor in proportion-to the squareofi the primary turns, so that the total reactanceoii the motor is increased at starting, asdesired. The objection tQbhl S PI'aCtlCQ'lS that it increases the primary resistance, thereby reducing the-fullload speedand the efficiency and, sometimes, the power factor oi the motor under running conditions Y According tomy invention, I overcomethe detrimentalfeilect or this reduction in theslot-inductance or the bar, at starting, by the use ofi' a slitted-b-ridgefi which hast/he following roperties. If the slit-width is w and the slit-depth t, the slot-factor for the slitted portion of the bridge, (which deter minesthe-slot-reactance of this" portion of'th-esee nrlpso g siotasa,which.

defines thefull -load leakage reactance across the portion of the SlOt OCCllPlBdf by the c o 11- dilator-bar 5, isg The total slot-reaotance,

at load; is thus. proportional to. bein determin d l y y. he ape f the slot, and regardless of the. material; of. the conductor bar 5. It willbe noted, however, that. he i ct e i i a l e -by. th first. his to: dq s'no ry at indicatedby the second factor. is redu ed by, say, 50% at' starting.

According.- to my invention, 13 make the first factor Eduction. in v the inductance, of th e secondfac or ats a a hel anla l. 1 2:: tion of the total inductance, thereby a ppreaching thecondition; otz'aa motor-having a starting secondary inductance approximately starting, whereas. that as high as the secondary running inductance.

It will be noted from the drawing and from the previous description of the slit 7 in the bridge 6 over each slot that there is a small iron bridge-portion 8 over each of the slits I. In. order to explain this, it is necessary to refer to the manner in which the rotor punchillgS. 3. are prepared and assembled.

The accurate assembly of the rotor punchings is not as easy" as the accurate assembly of the stator 'pun'chings which are built on an expanding arbor and are thus very accurately assembled without much trouble oreX- pense. Therotor punchings are assembled, however, on a shaft of relatively small diametenandl it. is. not economical to go to such extreme refinements. of assembly as will ensure a reasonably smoothouter periphery of the finished rotor, so that it is customary to. turn down the periphery-after the rotor is finished. and mounted on its shaft. It is fre-- quently desirable, as in the. motor which is shown in, Fig. 1-, to. utilize, for the rotor punc'hings, the discs which are punched out of: the inner. peripheries of the. stator punchings, so thatthe turning down of the finished rotor has. also. the function of: determining the depthof the, air-:gap, which may be some thing like twentyons thou-sandths of an inch on each sidev of they rotor.

It is-not practicable to-turn down a rotor. in which the slits 7. extend all the. way throughrto. the outer periphery of the rotor, asthetoollcatches on the slits and'causes very considerabletrouble. According to design, the distance above the slit may be some,- thing ofthe; order. ofia few hundredths of an inch, sot-hat, after the. rotor is turned down, as indicated by the broken line in Fig. 1,.the depth. of metal;.bctween. the top of the. slit; and: the outer periphery'shall; be about. ten; thousandths Olfii an inch, g

It is. desirable that thismetal: shall be, thin as possible because it. conducts. leakage flux and; under. the heavy startingcurrent conditions,.it: becomes saturated and therebyintroduces. a. small but appreciable harmful effect, adding ahoutO-n to, the slot factor. Iii this metal bri lge made to have depth less than tenthousandths or an; inch, the cutting tool; dips down; and. catches. in. it. duringthe turning V operation, as described above. It is believed that this innovation of a 10-111'11. ironbridge over. a slijtted bridge. of a machin ed rotor-member. is. an important feature ofmy contribution to the art;

It will be notedthat the bar-width b has nov effect upon the manner in which the reactance andthe resistance vary from starting to full speed but that it has. a, direct effect uponthe portion of the slot-inductance resulting from the transverse flu; crossing 'the'bar, As this is the'variable portion or the slot-inductance. it is desirable to makeit as small as possible,

or, in other words, to make the slot width b l bars are located. The total depth of slot,

that is, the distance of the bottom of the bar 5 from the outer periphery of the rotor, is also strictly limited by the necessity for avoiding saturation of the iron underneath the slot, as Well as intheteeth between the V able width,

slots.

It is an important feature of my invention, therefore, to make the slits 7 as narrow as possible, because the effect of the slitted bridge 6 is inversely proportional to the thickness of the slits, or, in other words, the depth t of the bridge, for agiven constant reactance effect, is inversely proportional to the slitwidth to, so that, if the slit is of any considerthe effect of any practicable bridge-depth which could be utilized, in a commercial deep-bar-rotor slot, would be quite small and unimportant. 7

While my invention is not altogether limited to the'material of the rotor bars, I have so far found that the practical commercial designs require the use of copper bars which are most readily obtained as harddrawn copper bars of rectangular-cross-section. Cast aluminum, which usually has a resistivity ofv approximately twice that of copper, and brass, having. a resistivity of approximately four times that of copper, would both require a greater slot depth and a greater space in which to put the rotor bars than are available in any,commercial designfor the class of service for which mymotor is primarily intended, as outlined hereinabove. V

I prefer to employ bars 5 and slots 4 of substantiall rectan ularsha e as bein the most practicable, for the following reasons. If the sides of the bars and of the slots 4 are made radial, the bars would be of wedgeshape, with a slightly greater thickness at 50 1 116 top than at the bottom, but they would still be substantially rectangular, since the depth of the bar is something of the order of one-quarter of the radius of the rotor, so that the thickness of the lower edge of the bar could not be less than about of thickness of the outer edge of the bar. Whilea bar of the form just mentioned would have more conductor material than the bar shown in Fig. 1 of the drawing, a conductor having a wider top than bottom would have less change in its resistance, as well as initsinductance, on starting, than a strictly rectangular bar, so that a deeper bar depth would be required in order to bring the resistance- --ratio up tothe value obtainable with a recgreat thickness 5 tangular bar, if the conductor were materially wider at the top than at the bottom. With a bottom which is three-fourths as wide as the top of the bar, the formula inner peripheries of the slots, and, as this minimum thickness of; tooth cannot be reduced, it would mean either a larger and more expensive motor or less material in the rotor conductors, and, hence,-too great a resistance during running conditions, which would mean poor performance, that is, a low speed and a poor ei'i'iciency, withalmost certain overheating in the rotor. Here again, it is obvious that no material departure from a rectangular bar cross-section is possible,

and, if the thickness of the bar at the top is of the thickness of the bar at the bottom,

the formula V is again accurate within 10%,if the thickness Z2 is measured at the top of the bar, as before. Y

Cylindrical conductors andslots-are out of the question in a squirrel-cage 'motor in which any material eddy-current elfect is desired. The slot factor is .625, regardless of its diameter, but the material of the conductor bar is so placed as to require a very and depth (Z, resulting in extreme saturation of the teeth and of the iron core below the conductors. In short, it is not commercially practicable'to make an induction motor utilizing the eddy-current effect to secure a ood startin tor ue with a cylindrical slot construction.

Certain practical limits may be noted.- Thus, with hard-drawn copper bars 'oper ating at (lat full load, the resistivity of the bar material is about .84X10G ohms per inch cube. In a sixty-cycle, copper-bar motor, the abscissas X in Fig. 2 would have to be multiplied by 100012, in order to obtain the bar depth d in inches." Thus, a-bar having a starting resistance three times its running resistance would haveapproximate 1y a bar depth of 9100 .00012=1.1 inches. It is believed that this calculation will be suilicientto illustrate the use of the curves in Fig. 2.

In general, it maybe said that the ratioof secondary or squirrel-cage resistances, at

starting, as compared with running, of a satisfactory motor of the type to which my invention is at present applied should be approximately 3 or more, or, at least, greater than 1.8 or 2. The reactance in the bridge portion of the slot should be at least approximately as large as the slot-reactance below the bridge at running speeds, or it should be such as to bring the ratio of the total slotshall be at least about 1200; and that or,in extreme cases, it may be as low as 6,000. For a copper-bar motor operating at a frequency other than 60 cycles,

that

The capacity-range of the motors of which I am primarily considering, in describing my invention, is from horsepower to 100 horsepower, in l-pole and 6-pole designs, or,

in general, the range of sizes is something of the order of 5 to 300. horsepower capacity.

' The bar-depth will remain fairly constant,

about one-third of d "tion of the'bridge, shall be well over 41 alat preferably 1% to 1 inches, regardless of the horsepower although, possibly, in motors of the larger sizes, it may sometimes be prace ticable to utilize a somewhat greater bardepth. Expressed in formula form, I should say that the bar-depth should be at least such that where P is the capacity in horsepower.

As to the physical dimensions of the slit in the bridge, I should say thattlie slitdepth t should be so great, and the slit-width 10 so small, that $6 is at least as large as I- prefer that the ratio representing the slot-factor of the'slitted porthough, in extreme. cases, I believethat less desirable but still useful results would be obtained with a slot-factor as low as 2, or

ciples of my invention and to delimit the quantitative values-of the novel features of my motordesigii.

it may be said I realize, of course, that certain changes and departures from my preferred designs may be made without departure from the essential spirit andresults of my invention, and I have attempted to explain the scope and nature of some of the said changes. I do not desire to be limited to my exact design except as specified in the appended claims when read in the light of the prior art.

I claim as my invention:

l. A sixty-cycle squirrel-cage induction motor having substantially rectangular deep bars of such depthd (in inches) as compared with their resistivity p (in ohms per inch cube) that the ratio of cl to w P is at least about 1200, characterized by having sl-itte'dbridge's above the squirrel-cage bars, and characterized further by having such bar-width b, slit width to and slit depth t that H) is at least as large as about one third of clw.

2. A squirrel-cage induction motor having.

substantially rectangular deep bars of such depth (Z (in inches), as compared with their resistivity p (in ohms per inch cube) and the line frequency. (in cycles per second), that p characterized by having slitted bridges above the squirrel-cage bars, and characterized fur ther by having such bar'width b, slit width to and slit depth t that t?) is atleast as large as about one third of dw;

3. A squirrel-cage induction motor having substantially rectangular deep' bars of such depth d (in inches), as compared with their resistivity p (in ohms per inch cube) and the line frequency (in cycles per second), that (1y? 6,000, characterized by having slitted bridges above the squirrel-cage bars, and character'- ized further by having such barwidth b, slit width w and slit depth" t't'hat' it is at least as large as about one third of clw.

4-AI1 iron-core, copper-bar, sixty-cycle,

squirre1-cage induction motor having sub-' stantially rectangular bars at least 1 inch deep, characterized by having slitted bridges above the squirrel-cage bars, the slit having such width '10 and depth t that the ratio is greater than 4.

5. An iron-core, copper-bar, sixty-cycle squirrel-cage induction motor having substantially rectangular bars at least. 1 inch deep,

above the squirrel-cage bar s,the slit having characterized by having slitted bridges 5 INF such Width 'w and depth t that the ratio is greater than 2.

6. An iron-core, copper-bar, squirrel-cage induction motor characterized by-having such bardepth (Z (in inches) as compared with the line frequency (in cycles per second) and capacity P (in h0rsepoWer),that

and characterized further by having slitted bridges above the squirrel-cage bars, the slit having such Width w and depth that the t ratio ,7) 18 greater than 3P 1/20,

7 The invention, as defined in claim 3, characterized by the fact that the slits in the bridges are closed at the air-gap periphery by an iron core-portion having a depth of V the order of 10 mils.

8. The invention, as defined in claim 4, characterized by the fact that the slits in the bridges are closed at the air-gap periphery by an iron core-portion having a depth of the order of 10 mils.

9. The invention, as defined in claim 6,. characterized by the fact that the slits in the bridges are closed at the air-gap periphery by an iron core-portion having a depth of the order of 10 mils.

In testimony whereof, I have hereunto subscribed my name this 18th day of October,

FRANCIS J. JOHNS. 

